The present invention relates to ionic liquids which are useful for cooling in high temperature environment.
According to generally accepted literature an ionic liquid is a salt in the liquid state, more particularly a melt of a low melting salt, e.g. with a melting point equal or below 100° C. (see e.g. Wasserscheid, Peter; Welton, Tom (Eds.); “Ionic Liquids in Synthesis”, Wiley-VCH 2008; ISBN 978-3-527-31239-9). However, it is to note that the melting temperature of ≤100° C. is chosen arbitrarily.
Such ionic liquids may exhibit some very interesting characteristics, e.g. having a very low, virtually non measurable vapor pressure, a large liquidus range, good electrical conductivity and interesting solvation characteristics. These characteristics make ionic liquids prone for several applications, e.g. as solvents (for example, in organic or inorganic synthesis, transition metal catalysis, biocatalysis, multiphase reactions, photochemistry, polymer synthesis, and nanotechnology), extracting agent (e.g. liquid-liquid or liquid gaseous extraction, sulphur removal during crude oil processing, removal of heavy metals during water processing and liquid membrane extraction), electrolytes (for example, in batteries, fuel cells, capacitors, solar cells, sensors, electroplating, electrochemical metal processing, electrochemical synthesis, and nanotechnology), lubricants, gels, reagents for organic synthesis, in the so-called “green chemistry” (e.g. as replacement for volatile organic compounds), antistatic addtives, specific applications in chemical analysis (e.g. gas chromatography, mass spectroscopy, capillary zone electrophoresis), liquid crystals, for storing and releasing hydrogen, as thermofluids, e.g. as cooling medium, etc..
In US 2009/314460 a process for strip casting is described using a travelling mould which is cooled by a liquid coolant, wherein the coolant is liquid metal or ionic liquid wherein ionic liquids are defined as a group of salts composed of organic cations and mostly inorganic anions which generally have a melting point below 100° C.
In WO 2010/136403 ionic liquids for use as a cooling medium are disclosed. It is described that ionic liquids are exclusively composed from ions (cations and anions) and are salts that are liquid at temperatures below 100° C. without the salts being dissolved in a solvent such as water. Cations according to WO 2010/136403 include imidazolium, pyridinium, pyrrolidinium, guanidinium, uronium, thiouronium, piperidinium, morpholinium, phosphonium or ammonium, which cations additionally can be alkylated and anions include sulfates, phosphates, halides, fluorinated anions such as tetrafluoroborate, hexafluoroborate, trifluoroacetate, trifluoromethanesulfonate and hexafluorophosphate, sulfonates, phosphinates or tosylates.
It is known that ionic liquids do have virtually no vapor pressure and are therefore generally non flammable below their high thermal decomposition point of up to 300° C. and even more. When heated up above their thermal decomposition temperature, however, they form gaseous, molecular decomposition products, which are flammable. In combustion experiments it can be seen, that a typical ionic liquid starts to burn after the bulk phase has reached the flashpoint temperature and that in many cases the combustion only continues, if a quite high input of external heat from a heat source is given. This is in contrast to conventional, molecular liquids: E.g. mineral oil can be ignited at temperatures of approx. 80 to 100° C., which is far below its thermal decomposition point, because mineral oil does have a vapor pressure and forms a flammable gas phase at this temperatures. As described in WO2010136403A1, ionic liquids do not form a highly explosive mixture of hydrogen and oxygen (detonating gas or oxyhydrogen gas) when in contact with hot (reducing) surfaces or hot (reducing) melts at temperatures above approx. 500° C. That is in contrast to water, which still is widely used as cooling agent. Drawbacks of ionic liquids in contrast to water may be the higher viscosity in the range of typically some 10 to some 100 mPas at 20° C. and a specific heat capacity of approx. 50 to 75% of water.
In summary, ionic liquid cooling media generally are superior to water or thermo oils as cooling agents in terms of safety. However, if ionic liquids are heated above their thermal decomposition point, they still form flammable or non flammable gaseous products, which will lead to an increase or even hazardous increase of pressure in a closed cooling system. In the case of an accidental efflux by e.g. disruption of a pipe into e.g. a molten metal it will cause heavy sputtering or even minor explosions.